Optical magnifiers are lens assemblies, typically in the form of a magnifying eyepiece, which are used to allow direct viewing of small items, such as electronic solid-state micro displays. For example, conventional micro-displays can range in resolution from 640×480 pixels to 1920×1200 pixels or higher in a viewing area of less than 20 mm across. For these displays to be easily viewed, they must be magnified. Thus, devices that incorporate one or more micro-displays will typically include a magnifier for each display. A magnifying eyepiece can be stand-alone and removable from the display device or integrated into the larger assembly. Typical uses are viewfinder or other display system for devices such as still or video cameras, night-vision systems, 3D display goggles, and other systems which incorporate small display panels.
To allow for easy viewing at a distance from the eyepiece, a magnifying eyepiece should have a relatively large eye relief, such as at least 18 mm. to allow for sharp viewing without the need for the user's eye to be precisely centered along the optical axis, thereby accommodating the viewer's lateral head motion relative to the magnifier. The magnifier should also have a wide field of view, such as greater than about +/−15 degrees. It is also important to minimize optical distortions across the entire field of view. These features must also be balanced with the overall cost of the eyepiece, generally determined by a combination of the type of glass used for each for each of the lenses, the difficulty in fabricating the lenses themselves, and the size of the lens. For example, lens costs are greater when a lens is fabricated with an aspheric or diffractive surface. Lens cost also increases when exotic glass is used.
A conventional eyepiece that can be used as an optical magnifier is a Plossl-type design. One type of Plossl design is disclosed in U.S. Pat. No. 4,482,217. As shown in FIG. 1A hereto, the Plossl lens design 10 of the '217 patent is comprised of two symmetric achromatic doublets 12, 14 comprised of a dense flint glass (Refractive index Nd=1.667 and Abbe number ν=33) and a crown glass (Nd=1.658 and ν=51). FIG. 1B is a through focus spot diagram for an exemplary 26 mm Plossl lens like that in the '217 patent using lens elements scaled to a 30 mm focal length. For a telescope, a 30 mm eyepiece using an f/6 objective would yield a 5 mm diameter exit pupil located at a distance from the eye lens known as the “eye relief.” The observer must place his eve pupil coincident with the eyepiece exit pupil to achieve full illumination and field of view. However, when used as a magnifier, there is no defined exit pupil or eye relief. This allows the observer to shift his eye laterally, imposing the necessity of far greater optical corrections. In this example, a large 10 mm entrance pupil is considered at a viewing distance of at 20 mm eye relief to illustrate the effect of lateral eye movement of a typical user with a 3 mm eye pupil (which is a typical sized pupil in daylight).
FIG. 1B is a through focus spot diagram showing spot sizes for a green wavelength of 0.588 um. The through focus spot diagram indicates the resolution over a field of view covering the eve shift within a 10 mm entrance pupil. It is desirable to minimize the spot size at the both the center and the edge of the field. As can be seen in FIG. 1B, this Plossl design provides a field of view up to 20 degrees off-axis. A user can see sharp images near the center of the field when the eye is well centered over the central axis of the lens. However, there is still noticeable off-axis aberration, increasing significantly upon reaching 20 degrees off-axis.
One way to quantify performance of a magnifier is to consider the RMS resolution values of the through focus spot diagram relative to typical visual acuity since aberrations that cannot be seen do not impact visual performance. A person with 20/20 eyesight has a visual acuity of 1′ (one arc minute), meaning that they can discern features as small as one arc minute across. A person with 20/40 eyesight, still reasonable, has a visual acuity of 2′. The Plossl magnifier illustrated herein has an on-axis resolution of 4.8′, a 10 degree off-axis resolution of 5.3′ and a 20-degree; off-axis resolution of 13.8.
Alternative ways of illustrating lens aberrations are shown in the graphs of FIGS. 1C-1D, which show field curvature and distortion, and longitudinal aberration, for this conventional lens at the same 0.588 um light wavelength. The divergence between the tangential and sagittal lines in FIG. 1D indicates astigmatism. The astigmatism is also indicated by the diamond shape of the focus spot diagram of FIG. 1B.
More complicated magnifier designs have been developed. However, these designs also suffer from various defects. Even conventional wide-field designs suffer from undesirable off-axis edge aberration effects for wide fields. These can be particularly detrimental in applications such as magnifying of displays that require high resolutions to be seen at the edge of the field. Others lens designs compensate for aberrations but do so by using exotic glass which can be very expensive and difficult to fabricate, or by incorporating aspheric and/or diffractive lens elements, features that can also increase fabrication cost and physical variations during fabrication.
Accordingly, it is an object to provide an economical optical magnifier that produces a substantially aberration free magnified image of a display over a wide field of view of at least +/−15 degrees, with good eye relief for ease of on and off-axis viewing of the display, and which can accommodate lateral head/eye motion without degrading resolution.
It is a further object of the invention to provide such an optical magnifier with an aberration free magnified image of a display over a very wide field of view and which can be fabricated using commonly available and relatively inexpensive types of glasses.
Yet a further object of the invention is to provide such an optical magnifier in which lens surfaces are spherical or flat, thereby reducing production costs and decreasing the likelihood of performance variations during production.